1. Field of the Invention
The present invention generally relates to an optical scanning apparatus that includes, for example, a laser source for irradiating a laser beam and a mirror for reflecting the laser beam.
2. Description of the Related Art
As one related art example, there is known an optical scanning apparatus that performs image projection by irradiating a laser beam to a mirror and scanning the laser beam by reflecting the laser beam to a desired direction.
FIGS. 9A-9B are schematic diagrams illustrating an example of an optical scanning apparatus according to a first related art example. FIG. 9A is a plan view illustrating an entire configuration of the optical scanning apparatus according to the first related art example. FIG. 9B is a schematic diagram illustrating a projection plane of the optical scanning apparatus according to the first related art example.
In FIG. 9A, the optical scanning apparatus of the first related art example includes a laser source 110, a collimator lens 120, and a mirror 150. First, a laser beam (laser light) L11 is irradiated from the laser source 110. Then, the laser beam L11 is turned into parallel light by the collimator lens 120 and is guided to the mirror 150. The mirror 150 oscillates vertically (up/down) around a horizontal axis and horizontally (left/right) around a vertical axis. Thereby, the mirror 150 scans the projection plane 170 by reflecting a reflection light L12 (i.e. the incident laser beam L11 reflected by the mirror 150) vertically and horizontally to the projection plane 170.
With the optical scanning apparatus of the first related art example illustrated in FIG. 9A, the laser source 110 and the collimator lens 120 are to be positioned diagonally before (in front of) the mirror 150 in order for the laser beam L11 to be incident to the mirror 150 from a position diagonally before (in front of) the mirror 150. Therefore, the optical scanning apparatus of the first related art example including the laser source 110 and the collimator lens 120 has a problem of taking up a large amount of space and becoming excessively large.
Further, with the optical scanning apparatus of the first related art example illustrated in FIG. 9A, the reflection light L12 reflected from the mirror 150 could not be bilaterally symmetric. As illustrated in FIG. 9B, the reflection light L12 exhibits a projection plane having a distorted shape in a horizontal (left/right) direction. In order to form the projection plane into a rectangle without the distorted shape, it becomes necessary to correct the distorted shape by using software. Thus, correcting the distorted shape leads to a problem of increased manufacturing cost.
In order to solve this problem, an optical scanning apparatus including an optical system illustrated in FIG. 10 is proposed. FIG. 10 is a plan view illustrating an exemplary configuration of an optical system of an optical scanning apparatus according to a second related art example. In the optical scanning apparatus illustrated in FIG. 10, a prism 130 and a wave plate 140 are positioned before (in front of) the mirror 150. The prism 130 has a cuboid shape. The laser source 110 and the collimator lens 120 are positioned at the side of the prism 130. Owing to this configuration, all of the components of the optical scanning apparatus of the second related art example can be positioned in the vicinity of the mirror 150. Thereby, the size of the optical scanning apparatus of the second related art example can be reduced. Further, the laser beam irradiated from the laser source 110 can be reflected by the prism 130, so that the reflected laser beam is perpendicularly incident to the mirror 150.
FIGS. 11A and 11B are schematic diagrams illustrating the optical scanning apparatus of the second related art example. FIG. 11A is a schematic diagram illustrating an entire configuration of the optical scanning apparatus according to the second related art example. FIG. 11B is a schematic diagram illustrating a projection plane of the optical scanning apparatus according to the second related art example.
As illustrated in FIG. 11A, the laser beam L11 irradiated from the laser source 110 becomes a parallel beam by passing through the collimator lens 120 and is perpendicularly incident to an end face of the prism 130 located toward the laser source 110. Then, the laser beam L11 changes direction by being reflected at the inside of the prism 130 and becomes perpendicularly incident to the mirror 150. The beam L12 reflected from the mirror 150 forms a projection plane 171 that is parallel to the mirror 150 and the prism 130. Thereby, an image is displayed. It is to be noted that the waveplate 140 polarizes the laser beam L11 and the reflection light L12, so that the reflection light L12 reflected from the mirror 150 is transmitted through the prism 130 without being reflected by the prism 130.
As illustrated in FIG. 11B, the projection plane 171 of the optical scanning apparatus according to the second related art example has a rectangular shape without any distortion. Thereby, an image can be appropriately projected to the projection plane 171.
Other than the above-described scanning type projector, there is also a projector using a liquid crystal light valve. The projector using the liquid crystal light valve includes an illumination unit that irradiates an illumination beam, an optical modulation unit that modulates the illumination beam from the illumination unit, a reflection type light attenuation filter that is positioned before (in front of) the optical modulation unit and attenuates the strength of the illumination beam incident to the optical modulation unit, and a retaining unit that retains the light attenuation filter in a position inclined at a predetermined angle relative to an optical axis of the illumination beam (see, for example, Japanese Laid-Open Patent Publication No. 2005-141152).
However, the optical scanning apparatus of the second related art example has a problem of stray light being generated at the center of the projection plane 171.
FIG. 12A is a perspective view for describing problems of the optical scanning apparatus according to the second related art example. FIG. 12B is a plan view for describing problems of the optical scanning apparatus according to the second related art example.
As illustrated in FIG. 12A, the optical scanning apparatus of the second related art example has a problem of a point (spot) P generated by stray light L13 at a center area of the projection plane 171 formed by the reflection light L12.
In the optical scanning apparatus of the second related art example illustrated in FIG. 12B, the laser beam L11 propagating to the mirror 150 is reflected by the prism 130. Then, the laser beam L11 reflected from the prism 130 is transmitted through the waveplate 140 and reaches the mirror 150. Although the waveplate 140 includes an end plane 140E having an anti-reflection film (not illustrated), approximately 0.5% of the laser beam L11 is reflected at the end plane 140E. The laser beam L11 reflected at the end plane 140E becomes the stray light L13 and is displayed as the point P at the center of the projection plane 171 regardless of the scanning operation of the mirror 150.
In a case of VGA (Video Graphics Array), the projection plane 171 has a resolution of approximately 300,000 dots (≈640×480) per screen. Accordingly, the light quantity of a single dot of a single screen per unit of time is 1/300,000 of the irradiated beam. In a case where the transmittance of the anti-reflection film (not illustrated) is 99.7%, the light reflected at the end plane 140E of the waveplate 140 has a light quantity of approximately 0.3% (approximately 1/300) of the irradiated beam. In other words, the light quantity of the stray light reflected from the end plane 140E is 300,000/300 (=1,000 times) of the scanning light (reflection light L12). The stray light having such light quantity is a problem that cannot be ignored.
It is to be noted that, unlike the scanning type projector, the above-described projector disclosed in Japanese Laid-Open Patent Publication No. 2005-141152 uses a liquid crystal light valve and includes components such as a light attenuation filter. Thus, the projector disclosed in Japanese Laid-Open Patent Publication No. 2005-141152 can take countermeasures for preventing stray light by using the components. On the other hand, a scanning type projector has fewer components compared to the above-described projector disclosed in Japanese Laid-Open Patent Publication No. 2005-141152 and has fewer components for preventing stray light. Thus, it is difficult to apply the projector disclosed in Japanese Laid-Open Patent Publication No. 2005-141152 to the below-described embodiments of the present invention.